Keira Knightley's pout worries

The 'Atonement' star believes her trademark facial expression makes it harder for her to convey varied emotions on the big screen.

She said: "The pout means I'm being c**p, like my face is frozen in this one look of surprise, and that's hardly helpful for conveying complex emotions. I think I was really pouty when I was 16 or 17 and that's when my first films came out, and it's sort of stuck.

"When I'm nervous my neck gets really, really tense and then that pressure sort of squeezes up to my lips and they push out and well, there you go, that's the pout."

The 23-year-old actress admits it is her ambition to prove herself as a serious actress.

She said: "Since my first films, like 'Bend it like Beckham', I've always been looking to be stretched - it doesn't always mean I'm going to be good, but I'm trying to be a good actress, I really am."